1. Field of the Invention
This invention relates to high-performance multilayer composite hollow fibers including at least one nonporous membrane layer and suitable for the separation of gases and for other purposes, as well as a method of making the same.
2. Description of the Prior Art
A large number of methods for the separation and purification of substances have been developed and improved from long ago.
The membrane separation technique is one of these methods. On a broad survey of the progress of its improvement, the general trend of technological advancement involves the development of excellent membrane materials, the development of techniques for forming thin membranes serving to enhance separating efficiency and the development of hollow fibers capable of enhancing equipment efficiency.
Among various separating membranes are nonporous membranes useful for the separation of gases and for other purposes. In such a nonporous membrane, the permeation rate for a gas is determined by its diffusion through the membrane, and the diffusion rate of the gas is greatly influenced by the thickness of the membrane. Accordingly, it is common practice to make the nonporous membrane as thin as possible. Moreover, since such a thin nonporous membrane has inadequate strength, attempts have been made to form a composite structure by combining the membrane with a porous layer. As one of such techniques for the formation of a thin membrane, a method is being extensively employed in which a thin membrane is formed on a porous substrate according to the coating or vapor deposition process. However, when a coating material is applied to a porous substrate, it penetrates into the pores of the substrate and fails to form a substantially thin membrane. More specifically, the membrane is sufficiently thin in the regions not corresponding to the pores of the porous substrate, but is undesirably thick in the regions corresponding to the pores. If an attempt is made to overcome this disadvantage by reducing the thickness of the membrane in the regions corresponding to the pores, pinholes will appear. For this reason, it is practically impossible to form a thin membrane of uniform thickness according to this method.
In order to overcome the above-described disadvantage, another method has been proposed in which the pores of a porous substrate are filled with a soluble material in advance, a thin membrane layer is formed on the surface of the substrate, and the soluble material is then leached out of the substrate. However, this method can hardly yield a thin membrane layer of uniform thickness. Moreover, this method is disadvantageous in that the thin membrane layer is liable to be damaged during the leaching process and in that the thin membrane layer tends to peel away from the finished composite membrane. Furthermore, it is difficult to apply this method to the manufacture of hollow fibers.
Still another method for forming a thin separating membrane is the formation of an asymmetric membrane from a polymer solution. For example, reverse osmosis membranes formed of aromatic polyamide and ultrafiltration membranes formed of polyacrylonitrile are being commercially produced by this method.
However, all of these membranes are formed according to such a technique that, in forming a membrane from a polymer solution, the superficial part of the membrane is solidified densely and the internal part thereof is made porous by selection of proper solidifying conditions or by use of the leaching process. Thus, these separating membranes consist of a single material.
Accordingly, the structure of the membranes formed by this method changes continuously from the superficial dense part toward the internal porous part and includes an intermediate structural part performing no important function. This is not so desirable from the viewpoint of filtering efficiency.
Moreover, the thin, nonporous membrane layer performing a separating function is exposed on one side of these composite membranes. This is disadvantageous in that any mechanical force exerted during manufacture or use tends to result in pinholes or cause damage to the nonporous membrane layer.